This invention relates to an exhaust-gas purification catalyst to be used close to the engine, for the purification of the exhaust gases from internal combustion engines, which comprises palladium on aluminum oxide and of barium oxide, as well as a process for its production. The catalyst exhibits a high activity and long-term stability to high temperature stresses.
For about three decades, exhaust-gas purification catalysts have been used in order to convert the pollutants emitted by combustion engines, namely carbon monoxide (CO), hydrocarbons (HC) and oxides of nitrogen (NOx), into the environmentally acceptable compounds water, nitrogen and carbon dioxide. All three pollutants can be simultaneously removed from the exhaust gas of internal combustion engines by means of so-called three-way catalytic converters if the engine is operated using stoichiometric air/fuel mixtures.
The actual catalyst composition consists for the most part of an oxide support having a very large surface area, onto which the catalytically active components are deposited in a very finely divided state. The precious metals of the platinum group—platinum, palladium, rhodium, iridium, ruthenium and osmium—are particularly suitable as catalytically active components for the purification of stoichiometric exhaust gases. Suitable supports are, for example, aluminum oxide, silicon dioxide, titanium oxide, zirconium oxide and mixed oxides and zeolites thereof. The use of so-called active aluminum oxides having a specific surface (BET surface area, measured in accordance with DIN 66132) of more than 10 m2/g is preferred. To improve the dynamic conversion, three-way catalytic converters contain in addition so-called oxygen-storing components. These include cerium oxide, praseodymium oxide and cerium/zirconium mixed oxides.
The form in which the components of the catalyst are used during its production has an appreciable influence on the subsequent function of the catalyst. In this invention, the following cases are distinguished:
a) as “Finely Divided Solid Substances”
                By these are meant pulverulent materials having particle sizes of between 1 and about 50 μm. In the literature published in English, the terms “bulk material” or “particulate material” are used to describe these.b) in the Form of Soluble “Precursor Compounds”        The precursor compounds are usually deposited onto solid substances having large surface areas and, by means of heat treatment in an oxidizing or reducing atmosphere, are converted into the actual components which promote catalysis.        
The catalyst composition is usually deposited in the form of a coating onto monolithic, inert honeycomb carriers made of ceramic or metal. The honeycomb carriers contain flow ducts for the gas being purified; these lie parallel to the longitudinal axis of the honeycomb carriers and are disposed over a narrow section of their cross-sectional area. The cell density (number of flow ducts per cross-sectional area of the honeycomb carriers) is generally between 10 and 200 cm−2. At the present time honeycomb carriers having cell densities of 62 cm−2 are still most frequently used. The catalytically active coating is deposited onto the wall surfaces of the partitions forming the boundary of the flow ducts in concentrations of 50 to 300 grams per liter (g/l) volume of the honeycomb carriers.
The known three-way catalytic converters are capable of adequately purifying the exhaust gases from internal combustion engines during normal operation. The residual emissions still remaining consist substantially of unburnt hydrocarbons, which are emitted mainly during the initial one to three minutes after the cold start. To decrease the cold-start emissions, so-called starter catalysts have been developed; these catalysts, unlike the normal three-way catalytic converters, are disposed not in the underbody region of the motor vehicle but close to the engine, in order to achieve as rapid a heating as possible of the starter catalysts to above their light-off temperature of 150 to 250° C. On the other hand, this arrangement impedes the starter catalysts during the normal operation of the motor vehicle, where exhaust-gas temperatures are up to 1000° C. and above. Starter catalysts, in the literature published in English also referred to as “close-coupled catalysts”, should therefore have a very good temperature stability in addition to a low light-off temperature for the hydrocarbons. In this invention, catalysts which are used close to the engine are referred to as starter catalysts.
WO 97/23278 describes a starter catalyst which contains a metal of the platinum group on a support and has substantially no oxygen storing material. In a special embodiment of the starter catalyst, the latter contains palladium on aluminum oxide as well as an alkaline-earth metal to provide stability to thermal stresses. To deposit palladium onto the aluminum oxide, the aluminum oxide is impregnated with a solution of palladium nitrate. The palladium-containing aluminum oxide, together with a solution of zirconyl acetate and deionized water, is then ground. Solid barium oxide may also be added to the coating suspension.
An object of the present invention is to provide a starter catalyst comprising palladium, aluminum oxide and barium oxide and exhibiting an improved temperature stability compared with known starter catalysts.